Hydraulic pumps and motor are well known and widely used, having reciprocating pistons mounted in respective cylinders formed in a cylinder block and positioned circumferentially at a first radial distance about the rotational axis of a drive element. Many of these pump/motor machines have variable displacement capabilities, and they are generally of two basic designs: (a) either the pistons reciprocate in a rotating cylinder block against a variably inclined, but otherwise fixed, swash-plate; or (b) the pistons reciprocate in a fixed cylinder block against a variably inclined and rotating swash-plate that is often split to include-a non-rotating (i.e., nutating-only) “wobbler” that slides upon the surface of a rotating and nutating rotor. While the invention herein is applicable to both of these designs, it is particularly appropriate for, and is described herein as, an improvement in the latter type of machine in which the pistons reciprocate in a fixed cylinder block.
As indicated above, this invention is directed to “liquid” (as distinguished from “gas”) type hydraulic machines and it should be understood that the terms “fluid(s)” and “pressurized fluid(s),” as used herein throughout the specification and claims, are intended to identify incompressible liquids rather than compressible gasses. Because of the incompressibility of liquids, the pressure and load duty cycles of the these two different types of hydraulic machines are so radically different that designs for the gas compression type machines are inappropriate for use in the liquid-type machines, and visa versa. Therefore, the following remarks should all be understood to be directed and applicable to liquid-type hydraulic machines and, primarily, to such heavy duty automotive applications as those identified in the Technical Field section above.
Hydraulic machines with fixed cylinder blocks can be built much lighter and smaller than the machines that must support and protect heavy rotating cylinder blocks. However, these lighter machines require rotating and nutating swash-plate assemblies that are difficult to mount and support. For high-pressure/high-speed service, the swash-plate assembly must be supported in a manner that allows for the relative motion between the heads of the non-rotating pistons and a mating flat surface of the rotating and nutating swash-plate. As just indicated above, such prior art swash-plates have often been split into a rotating/nutating rotor portion and a nutating-only wobbler portion, the latter including the flat surface that mates with the heads of the non-rotating pistons through connecting “dog bones”.
That is, such fixed-cylinder-block machines have heretofore used a “dog-bone” extension rod (i.e., a rod with two spherical ends) to interconnect one end of each piston with the flat surface of the nutating-but-not-rotating wobbler. One spherical end of the dog bone is pivotally mounted into the head end of the piston, while the other spherical end is usually covered by a pivotally-mounted conventional “shoe” element that must be held at all times in full and flat contact against the flat surface of the swash-plate wobbler during all relative motions between the heads of the non-rotating pistons and a mating flat surface of the nutating swash-plate. As is well known in the art, these relative motions follow varying non-circular paths that occur at all inclinations of the swash-plate away from 0°. These dog-bones greatly increase the complexity and cost of building the rotating swash-plates of these lighter machines.
Dog-bone rods are also sometimes used to interconnect one end of each piston with the inclined (but not rotating) swash-plates of hydraulic machines with rotating cylinder blocks. However, more often this latter type of machine omits such dog-bones, using instead elongated pistons, each having a spherical head at one end (again, usually covered by a pivotally-mounted conventional shoe element) that effectively contacts the non-rotating flat surface of the swash-plate. Such elongated pistons are designed so that a significant portion of the axial cylindrical body of each piston remains supported by the walls of its respective cylinder at all times during even the maximum stroke of the piston. This additional support for such elongated pistons is designed to assure minimal lateral displacement of each spherical piston head as it slides over the inclined-but-not-rotating swash-plate when the pistons rotate with their cylinder block.
Generally, these elongated pistons are primarily lubricated by “blow-by”, i.e., that portion of the high pressure fluid that is forced between the walls of each cylinder and the outer circumference of each piston body as the reciprocating piston drives or is driven by high pressure fluid. Such blow-by provides good lubrication only if tolerances permit the flow of sufficient fluid between the walls of the cylinder and the long cylindrical body of the piston, and blow-by sufficient to assure good lubrication often negatively effects the volumetric efficiency of the pump or motor machine. For instance, a 10 cubic inch machine can use as much as 4 gallons of fluid per minute for blow-by. While smaller tolerances can often be used to reduce blow-by, the reduction of such tolerances is limited by the needs for adequate lubrication that increase with the size of the pressure and duty loads of the machine. Of course, such blow-by is accomplished by using fluid that would otherwise be used to drive or be driven by the pistons to accomplish work. Therefore, in the example just given above, the 4 gallons of fluid per minute used for blow-by lubrication, reduces the volumetric efficiency of the machine.
The invention disclosed below is directed to improving the volumetric efficiency of such elongated-piston machines while, at the same time, assuring (a) appropriate lubrication of the pistons and (b) simplification of the apparatus used to maintain contact between the pistons and the swash-plate.